At present, a discharge type fluorescent lamp and an incandescent bulb used as the illumination device involve problems that a harmful substance such as mercury is contained, and life span is short. However, in recent years, a high luminescence LED emitting light of near ultraviolet/ultraviolet to blue color has been developed in sequence, and the white LED illumination for the practical application of the next generation has been actively studied and developed, in which the white light is created by mixing the light of the near ultraviolet/ultraviolet to blue color generated from the LED and the light generated from the phosphor having an excitation band in a wavelength region thereof. When the white LED illumination is put to practical use, since efficiency of converting electric energy into light is improved, less heat is generated and it is constituted of the LED and a phosphor, the white LED has advantages of good life span without burn-out of a filament like a conventional incandescent bulb and the harmful substance such as mercury is not contained, and miniaturization of the illumination device is realized, thus realizing an ideal illumination device.
There is a system of white LED illumination for creating a white light by combining this high luminescence LED and the phosphor, called one chip type system, and in the one-chip type system, the white light having excellent color rendering properties can be obtained, compared to the multi-chip type system in which white color is created by using three primary color LED such as red LED, green LED, and blue LED of high luminescence. Further, cost reduction is realized by the one-chip type system. Therefore, the one-chip type system is focused as an illumination of the next generation.
For example, the white LED illumination of one chip type system is given, in which a high luminance blue LED and a phosphor excited by blue light emitted from this LED and emitting yellow light are combined, and for example, the high luminance blue LED using an InGaN-based material and a garnet-based yellow phosphor (Y, Gd)3(Al, Ga)5O12:Ce (YAG:Ce), Tb3Al5O12:Ce, Ca3Sc2Si3O12:Ce are combined. In this white LED illumination, complementary relation of blue and yellow of the light is utilized. Initially, although this white LED illumination has high luminance, light emission on the longer wavelength side of a visible light region, namely light emission of a red color component is insufficient, thus involving a problem of inferior color rendering properties, which is an important factor as illumination. However, in recent years, the phosphor having an emission peak wavelength in a range from yellow to red, with a broad peak in an emission spectrum, and the phosphor containing nitrogen having an excellent excitation band in a range from near ultraviolet/ultraviolet to blue color have been sequentially developed, and by adding these phosphors, the color rendering properties are improved. For example, Ca2Si5N8:Eu, Sr2Si5N8:Eu, Ba2Si5N8:Eu, (Ca, Sr, Ba)2Si5N8:Eu, Cax(Al,Si)12(O, N)16:Eu (0<x≦1.5), CaSi2O2N2:Eu, SrSi2O2N2:Eu, BaSi2O2N2:Eu, (Ca, Sr, Ba)Si2O2N2:Eu, CaAl2Si4N8:Eu, CaSiN2:Eu, CaAlSiN3:Eu are typically given as examples of the phosphor containing nitrogen.
However, the white LED illumination, in which the high luminance blue LED and the garnet-based yellow phosphor are combined, has problems that the garnet-based yellow phosphor has no flat excitation band in the vicinity of the excitation band wavelength 460 nm, and there is a variation in emission intensity of the high luminance blue LED and the peak wavelength, and a balance of the emission intensity of blue and yellow is lost when the phosphor is applied on the LED because the emission intensity of transmitted blue light is changed depending on a film thickness, resulting in change of color tone of the white light.
In order to solve the above-described problem, study and examination on the white LED illumination system have been actively performed, in which white color is obtained, by utilizing mixing color of lights obtained from the LED emitting near ultraviolet/ultraviolet light, and the phosphor emitting red color (R), the phosphor emitting green color (G), and the phosphor emitting blue color (B), excited by the light of near ultraviolet/ultraviolet light emitted from this LED. This system is more excellent in color rendering properties, because an arbitrary emission color can be obtained in addition to white light, by combination and a mixing ratio of R, G, B, and the white emission is obtained by not the complementary relation of the light but by a mixing color relation, and by using R, G, B and other phosphor with broad emission spectrum, the emission spectrum close to sun light can be obtained compared with the illumination in which the high luminance blue LED and the garnet-based yellow phosphor are combined. Further, even when there is a variation in the emission intensity and the peak wavelength, as is seen in the high luminance blue LED, the near ultraviolet/ultraviolet ray is not utilized in mixing color of the light. Therefore, phenomenon such as changing the color tone of the white light is not allowed to occur, and the white LED illumination with excellent color rendering properties and no variation of color tones can be manufactured.
Then, as the phosphor used in this purpose of use, Y2O2S:Eu, La2O2S:Eu, 3.5MgO.0.5MgF2.GeO2:Mn, (La, Mn, Sn)2O2S.Ga2O3:Eu are given as examples of a red phosphor, and ZnS:Cu, Al, CaGa2S4:Eu, SrGa2S4:Eu, BaGa2S4:Eu, SrAl2O4:Eu, BAM:Eu, Mn, (Ba, Sr, Ca)2SiO4:Eu are given as examples of a green phosphor, and BAM:Eu, Sr5(PO4)3Cl:Eu, ZnS:Ag, (Sr, Ca, Ba, Mg)10(PO4)6Cl2:Eu are given as examples of a blue phosphor.
The aforementioned Y2O2S:Eu, La2O2S:Eu, 3.5MgO.0.5MgF2.GeO2:Mn, (La, Mn, Sm)2O2S.Ga2O3:Eu, ZnS:Cu, Al, CaGa2S4:Eu, SrGa2S4:Eu, BaGa2S4:Eu, SrAl2O4:Eu, BAM:Eu, Mn, (Ba, Sr, Ca)2SiO4:Eu, and BAM:Eu, Sr5(PO4)3Cl:Eu, ZnS:Ag, (Sr, Ca, Ba, Mg)10(PO4)6Cl2:Eu have a high efficient excitation band even in the near ultraviolet/ultraviolet region, with the emission spectrum having a broad peak. Therefore, the luminance and the color rendering properties are improved even in the white LED illumination in which the near ultraviolet/ultraviolet LED and R, G, B and other phosphors are combined. However, there is no high efficient and high luminance R, G, B and other phosphors such as a YAG:Ce phosphor used in a combination of the high luminance blue LED and the garnet-based yellow phosphor. Therefore, satisfactory white LED illumination can not be obtained.
Accordingly, a new phosphor having further excellent emission characteristics is developed, and regarding the blue phosphor also, a new blue phosphor exceeding the current BAM:Eu, Sr5(PO4)3Cl:Eu, ZnS:Ag, (Sr, Ca, Ba, Mg)10 (PO4)6Cl2:Eu has been actively performed, and in recent years, La1-xSi3N5:Cex (for example, see patent document 1) is reported as the phosphor containing nitrogen.
Also, inventors of the present invention reports the phosphor having a broad emission spectrum in a range of blue color (in a peak wavelength range from 400 nm to 500 nm) and having a broad flat excitation band in a range of the near ultraviolet/ultraviolet, and having excellent emission efficiency and emission intensity/luminance (see patent document 2).    Patent document 1: Japanese Patent Laid Open Publication No. 2003-96446    Patent document 2: Japanese Patent application No. 2005-380323